KR20140080346A - Method for manufacturing supper austenitic stainless steel - Google Patents

Abstract

The present invention relates to a method of manufacturing supper austenitic stainless steel having superior corrosion resistance. The method of manufacturing supper austenitic stainless steel having superior corrosion resistance according to an embodiment of the present invention includes the steps of performing strip casting and heat treatment of molten steel having an adjusted composition, performing intermediate rolling of the heat-treated strip, and producing a hot-rolled coil; cold-rolling the hot-rolled coil, and primarily annealing the cold-rolled coil to remove the remaining stress; raising the temperature of the cold-rolled coil, which has been subject to the primarily annealing process, to a target temperature; and secondarily annealing the cold-rolled coil while applying annealing tensile stress in the range of 2.0-3.0 kgf/mm^2 to the cold-rolled coil at the target temperature.

Description

[0001] METHOD FOR MANUFACTURING SUPER AUSTENITIC STAINLESS STEEL [0002]

The present invention relates to a method for manufacturing super-austenitic stainless steel, and more particularly, to a method for manufacturing super-austenitic stainless steel excellent in corrosion resistance.

Generally, stainless steel is classified according to chemical composition or metal structure. According to the metal structure, the stainless steel is classified into an austenitic system (300 system), a ferrite system (400 system), a martensitic system, and an ideal system.

Among these stainless steels, austenitic stainless steels are excellent in corrosion resistance and non-magnetic and are widely used in kitchen containers, heavy chemical industry, and building interior and exterior materials.

Particularly, super-austenitic stainless steel containing 22 to 24 wt% of Cr, 20 to 23 wt% of nickel, 6.0 to 6.8 wt% of molybdenum and 0.21 to 0.32 wt% of nitrogen (N) Steel (S32050) Steel has high resistance to corrosion due to its high resistance to corrosion and is used as a material for power plant, condenser tube and plate heat exchanger.

The super-austenitic stainless steels having excellent corrosion resistance have a high content of chromium, molybdenum and nitrogen, so that surface cracks occur frequently during hot rolling, and the rate of occurrence of these cracks is extremely low. In order to secure hot workability, superalloy stainless steels have a low sulfur content of 0.0003% (3 ppm) So that there is a problem that the refining time takes a long time.

The sigma (δ) phase is very easily generated in the temperature range of 900 to 1050 ° C., and when the cold rolled steels are continuously annealed, residual stress (χ) is further precipitated in the range of 700 ° C. to 800 ° C., There is a phenomenon of deterioration.

Accordingly, there is a demand for a control condition for avoiding surface cracking during hot rolling of super-austenitic stainless steels, and for suppressing the generation of intermetallic compounds (sigma phase, car disorder, etc.) in cold rolled steel and ensuring corrosion resistance.

Therefore, in the "thick austenitic stainless steel product and its manufacturing method (Patent Document 10-0045737; Patent Document 1)" as a method for controlling intermetallic compounds in the manufacture of super-austenitic stainless steels, the annealing temperature is from 1038 to 1150 Annealing heat treatment is carried out in austenitic steel and steel products (Japanese Patent Application Laid-Open No. 2007-0089971; Patent Document 2). In a high-alloy super-austenitic stainless steel, a nitrogen content of 0.5 % ≪ / RTI > to 1.1%.

However, as in Patent Document 1, there is a possibility that the annealing condition of the actual super-austenitic stainless steel is not heat-treated at only the temperature of 1038 ° C to 1150 ° C and the sigma phase is not decomposed to remain, It is difficult to manufacture a slab in an atmospheric pressure atmosphere due to the generation of pores at a high nitrogen content of 0.5% or more as in Document 2.

On the other hand, in a method of producing an austenitic stainless steel having excellent water resistance (Patent Document 10-0056581; Patent Document 3), a casting method in which the equiaxed retention ratio in the slab is controlled to 20% or less in order to suppress the precipitation of sigma phase during continuous casting And that the reduction of intermetallic compounds is not easy as a continuous casting method.

In addition, the slabs produced by the ordinary continuous casting process of the super-austenitic stainless steels have a solidification rate, the chromium and molybdenum components are segregated in the center portion, the sigma phase of about 100 탆 in the slab of the cast structure exists, A large number of sigma phases having a length of about 10 to 20 mu m remained and sigma phase decomposition was very difficult.

Patent No. 10-0045737 (Nov. Published Patent 2007-0089971 (2007.09.04) Patent No. 10-0056581 (Nov. 19, 1992)

In the present invention, a strip casting process in which a hot rolling process is omitted is applied to suppress hot rolled surface cracking, and an intermediate residual stress removal heat treatment is applied to a target annealing temperature, The present invention provides a super-austenitic stainless steel manufacturing method capable of avoiding sections as much as possible.

Further, the present invention provides a method of manufacturing super-austenitic stainless steel capable of controlling a sigma phase size generated by segregation during solidification to 3 mu m or less.

A super-austenitic stainless steel manufacturing method according to an embodiment of the present invention includes: strip casting molten steel with adjusted components, followed by heat treatment, and intermediate rolling (IRM) the heat-treated strip to produce a hot-rolled coil; A step of subjecting the produced hot-rolled coil to cold-rolling and then subjecting the cold-rolled coil to primary annealing to remove residual stress; Heating the cold-rolled coil subjected to the first annealing to a target temperature; And a step of subjecting the cold-rolled coil subjected to the first annealing at the target temperature to secondary annealing while applying an annealing tensile stress in the range of 2.0 to 3.0 kgf / mm < ^{2 &} gt ;.

Wherein the molten steel has a carbon (C) content of more than 0 to 0.030 wt%, a silicon (Si) content of more than 0 to 0.5 wt%, a manganese (Mn) content of more than 0 to 0.6 wt% P: more than 0 and less than 0.035 wt%, S: not less than 0.001 and not more than 0.004 wt%, Cr: more than 22 and not more than 24 wt%, Ni: more than 20 wt%, molybdenum (Mo) : More than 6.0 to not more than 6.8 wt%, nitrogen (N): not less than 0.21 wt%, balance iron (Fe) and inevitable impurities, and the value of the formula resistance index .

[Formula 1]

Formal resistance index (PREN) =% Cr + 3.3% Mo + 30% N

In Equation 1,% Cr,% Mo, and% N mean the content (wt%) of each component.

In the step of producing the hot-rolled coil, the strip is maintained at a temperature interval of 1230 to 1250 ° C for 15 to 60 seconds by induction heating before the intermediate rolling process.

The cold-rolled coil is maintained at a temperature range of 530 to 580 占 폚 for 2 to 5 minutes in the step of removing the residual stress by performing the primary annealing of the cold-rolled coil.

Wherein the step of raising the temperature of the primary annealed cold-rolled coil to the target temperature raises the temperature to a target temperature of 1180 to 1220 占 폚 while raising the temperature at a rate of 10 占 폚 / sec or more.

Wherein the step of raising the temperature of the cold-rolled coil subjected to the primary annealing to a target temperature is characterized in that the temperature raising rate is 20 占 폚 / sec or less.

Characterized in that the cold annealing coil subjected to the first annealing is held for 1 to 3 minutes at a target temperature range of 1180 to 1220 占 폚 in the second annealing treatment at the target temperature.

According to the embodiment of the present invention, when a super cast austenitic stainless steel hot-rolled coil is manufactured, a strip casting process is applied instead of a conventional continuous casting process, and in the case of performing cold annealing annealing, a residual stress removing intermediate heat treatment, The temperature and the tensile stress applied to the material during annealing can be suitably controlled so that the decomposition of the residual intermetallic compound is promoted by utilizing the stress organic transformation to secure the uniformity of the texture and at the same time to obtain the cold- .

FIG. 1 is a photograph of a sigma-phase distribution structure of a heat spreader manufactured by a strip casting process according to an embodiment of the present invention and a heat spreader (conventional example) manufactured by a general strip casting process,
2 is a graph showing the precipitation behavior of an intermetallic compound depending on temperature and time of super-austenitic stainless steel according to an embodiment of the present invention,
FIG. 3 is a graph showing the relationship between precipitation of intermetallic compounds (sigma phase, carbon dioxide) and corrosion resistance in super-austenitic stainless steels,
4 is a graph showing a good corrosion resistance section according to annealing temperature and tensile stress acting on a material of super-austenitic stainless steel according to an embodiment of the present invention,
FIG. 5 is a photograph showing the microstructure of a cold rolled steel product (according to the present invention) and a final cold rolled steel product (conventional example) of a general super-austenitic stainless steel manufactured according to an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. It will be apparent to those skilled in the art that the present invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, It is provided to let you know.

(P): more than 0 and not more than 0.035 wt%; (c) more than 0 wt% of carbon; (C) , Sulfur (S): more than 0.001 to less than 0.004 wt%, chromium (Cr): more than 22 to less than 24 wt%, nickel (Ni): more than 20 to less than 23 wt%, molybdenum (Mo) (N): greater than 0.21 to less than 0.32 wt%, the balance of iron (Fe) and unavoidable impurities, and having a value of the resistance index of 50 or more as defined by the following formula 1 do.

[Formula 1]

Formal resistance index (PREN) =% Cr + 3.3% Mo + 30% N

Here,% Cr,% Mo, and% N mean the content (wt%) of each component.

Super austenitic stainless steels are related to ASTM standard components of austenitic stainless steels and are known as S32050 steels. The composition ranges and the reason for limiting the composition ranges thereof are well known to those skilled in the art, so that a detailed description of the composition ranges will be omitted.

According to the present invention, molten steel whose component is adjusted for super-austenitic stainless steel, which is a target steel, is subjected to strip casting and heat treatment, and the heat-treated strip is subjected to an intermediate rolling (IRM) process to produce a hot-rolled coil.

In this case, it is preferable to maintain the strip at a temperature interval of 1230 to 1250 ° C for 15 to 60 seconds by induction heating before the intermediate rolling process.

FIG. 1 is a sigma phase distribution histogram of a heat spreader manufactured by a strip casting process according to an embodiment of the present invention and a heat spreader (conventional example) manufactured by a general strip casting process, As described above, the sigma phase is easily decomposed in the state of the art and remains very fine and small in the final thermal expansion state. However, even in the case of producing strips by strip casting, in the case of the existing method (existing) in which the heat treatment process by induction heating is not applied as in the present invention, there is a possibility that the coarse sigma phase remains and remains in the final cold- .

The hot rolled material subjected to the heat treatment by induction heating is subjected to cold rolling by a conventional method to produce a cold rolled coil.

Then, the produced cold-rolled coil is annealed, and the annealing process is divided into three sections.

First, the cold-rolled coil is maintained in the temperature range of 530 to 580 ° C for 2 to 5 minutes to perform primary annealing. The residual stress of the cold-rolled coil is removed by the primary annealing treatment.

FIG. 2 is a graph showing the precipitation behavior of an intermetallic compound depending on the temperature and time of a super-austenitic stainless steel according to an embodiment of the present invention. In FIG. 2, the intermetallic compounds, It can be seen that this occurs very quickly. However, when the intermediate heat treatment process for removing the residual stress is performed between 530 ° C. and 580 ° C., the residual stress generated during cold rolling is removed, and the deposition of the sigma phase and the carbide is delayed.

When the residual stress of the cold-rolled coil is removed by the primary annealing process, the cold-rolled coil subjected to the primary annealing is heated to a target temperature of 1180 to 1220 DEG C while raising the temperature at a rate of 10 DEG C / sec or more. In FIG. 2, it is shown that the risk of generation of a sigma phase and a cogging phase can be reduced when the rate of temperature rise of the cold-rolled coil is set to 10 ° C / sec or more. However, since increasing the temperature raising rate greatly decreases the economical efficiency, the lower limit value of 10 DEG C / sec is meaningful in the present invention, and when the economical efficiency is considered, the upper limit value is 20 DEG C / sec.

If the temperature of the cold-rolled coil is raised to the target temperature, it is maintained for 1 to 3 minutes at the target temperature range of 1180 to 1220 ° C. In this case, a line tension applied to the cold-rolled coil at a target annealing temperature is applied in a range of 2.0 to 3.0 kgf / mm ² to accelerate the decomposition of residual intermetallic compounds due to the stress-induced transformation to obtain a cold- Can be prepared.

FIG. 3 is a graph showing the relationship between precipitation of intermetallic compounds (sigma phase, phase transition) and corrosion resistance in super-austenitic stainless steels, and FIG. 3 is a graph showing the relationship between the rate of increase in temperature and the heat treatment temperature , The corrosion resistance is rapidly deteriorated.

[Example]

The following examples illustrate the present invention.

FIG. 4 is a graph showing a good corrosion resistance period according to the annealing temperature and the tensile stress acting on the material of the super-austenitic stainless steel according to an embodiment of the present invention, and FIG. This is a photograph showing the microstructure of a cold rolled steel sheet (example) and a final cold rolled steel sheet (conventional example) of a general super-austenitic stainless steel.

In the present invention, all the temperatures are based on the material temperature of the strip.

Table 1 below shows the characteristics of a hot austenitic super-austenitic S32050 steel produced by a strip casting process, followed by cold rolling and subsequent annealing of coils, and a cold- Is annealed and annealed.

division Casting conditions Cold annealing Quality characteristics Strip at casting
Heating condition
(Temperature / time) Residual stress removal
Primary annealing treatment
(Temperature / time) The heating rate (550 ^° C →
Annealing temperature (캜) Second annealing treatment
Annealing temperature (캜)
/ Hour (minute) Tensile stress
(line tension) Intermetallic
compound
Corrosion resistance Comparative Example 1 Unapplied 530 to 550 ^° C
/ 2 to 5 minutes 5 ℃ / sec or less 1180 ~ 1220 ℃
/ 1 to 3 minutes 1.5 kgf / mm ² X Bad Comparative Example 2 Unapplied 530 to 550 ^° C
/ 2 to 5 minutes 10 ° C / sec 1180 ~ 1220 ℃
/ 1 to 3 minutes 1.5 kgf / mm ² △ usually Comparative Example 3 1230 ~ 1250 ℃
/ 15 ~ 60 seconds Unapplied 10 ° C / sec 1180 ~ 1220 ℃
/ 1 to 3 minutes 2.0 kgf / mm ² △ usually Comparative Example 4 1230 ~ 1250 ℃
/ 15 ~ 60 seconds 530 to 550 ^° C
/ 2 to 5 minutes 10 ° C / sec 1140 to 1180 ° C
/ 1 to 3 minutes 3.0 kgf / mm ² △ usually Comparative Example 5 1230 ~ 1250 ℃
/ 15 ~ 60 seconds 530 to 550 ^° C
/ 2 to 5 minutes 10 ° C / sec 1180 ~ 1220 ℃
/ 1 to 3 minutes 3.5 kgf / mm ² ◎ Plate break
Occurrence (★) Inventory 1 1230 ~ 1250 ℃
/ 15 ~ 60 seconds 530 to 550 ^° C
/ 2 minutes 10 ° C / sec 1180 ~ 1220 ℃
/ 1 to 3 minutes 2.0 kgf / mm ² ◎ Good Inventory 2 1230 ~ 1250 ℃
/ 15 ~ 60 seconds 530 to 550 ^° C
/5 minutes 10 ° C / sec 1180 ~ 1220 ℃
/ 1 to 3 minutes 3.0 kgf / mm ² ◎ Good Conventional Example 1 Unapplied Unapplied 5 ℃ / sec or less 1140-1180 DEG C
/ 1 to 3 minutes 1.5 kgf / mm ² X Bad Conventional Example 2 Unapplied Unapplied 5 ℃ / sec or less 1180 ~ 1220 ℃
/ 1 to 3 minutes 2.0 kgf / mm ² △ Bad

As shown in Table 1, in the conventional example 1 and the conventional example 2, when the S32050 steel is manufactured by the continuous casting method or by strip casting, the hot-rolled coil is manufactured without performing the strip heat treatment process before the intermediate rolling (IRM) Slow heating rate and low tensile stress were applied to annealing annealing process.

Accordingly, in the case of Conventional Example 1 in which the annealing is performed for 1 to 3 minutes or less at a temperature of 1140 to 1180 占 폚, the residual sigma phase in the hot-rolled coil and the heating rate of 5 占 폚 / sec or less The resultant sigma phase is partially decomposed at the annealing temperature, but the additional stress organic transformation is not promoted due to the low tensile stress of 1.5 kgf / mm ^{2, and} the sigma phase remains and the corrosion resistance is lowered.

In the case of Conventional Example 2 in which the annealing is performed for a time within 1 to 3 minutes at a temperature of 1180 ° C to 1220 ° C (secondary annealing treatment), a heating rate of 5 ° C / sec or less and a low tensile force of 2.0kgf / mm ² The sigma phase remains as in Conventional Example 1 due to the stress and the corrosion resistance is lowered.

On the other hand, in Comparative Example 1, the residual stress relief heat treatment was first applied for a time period of 2 to 5 minutes at a temperature range of 530 ° C to 580 ° C at the time of cold rolling annealing, and then the target annealing temperature 1180 when a heat treatment for from 1 to less than 3 minutes at a temperature of ℃ ~ 1220 ℃, the cast strip during the intermediate heat treatment not yet executed, and the material between the metal compound at a lower tensile stress of 1.5kgf / mm ² applied to the residue and the corrosion resistance is lowered .

In Comparative Example 2, in the condition of Comparative Example 1, the rate of temperature rise from the residual stress removal heat treatment temperature to the target annealing temperature was as high as 10 DEG C / sec, and partial sigma phase formation was prevented. However, ² showed that the intermetallic compound remained and the corrosion resistance was lowered due to the low tensile stress.

In the case of Comparative Example 3, when the hot-rolled coil was produced after the strip was subjected to heat treatment at a temperature of 1230 ° C to 1250 ° C for 15 to 60 seconds or less during the casting, the rate of increase in temperature was 10 ° C / When the annealing temperature is 1180 ° C ~ 1220 ° C and the annealing process is applied to the cold-rolled coil for a time within 1 to 3 minutes under a tensile stress of 2.0kgf / mm ^2, the intermediate residual stress is removed. And the corrosion resistance is partially improved, but satisfactory results do not appear.

In Comparative Example 4, the strip was heat-treated at a temperature of 1230 ° C to 1250 ° C for 15 to 60 seconds or less during casting, and then a hot-rolled coil was prepared. The hot rolled coil was annealed at a temperature of 530 ° C to 580 ° C for 2 to 5 minutes After applying the residual stress relieving heat treatment for a period of time, tensile stress of 3.0 kgf / mm ² is applied to the cold-rolled coil for 1 to 3 minutes at the temperature of the target annealing temperature of 1140 ° C to 1180 ° C at a temperature raising rate of 10 ° C / In the case of the heat treatment, the intermetallic compound is slightly retained due to the low annealing temperature and the corrosion resistance is partially improved, but satisfactory results are not obtained.

In Comparative Example 5, a hot-rolled coil was produced by heat-treating the strip at a temperature of 1230 ° C. to 1250 ° C. for 15 to 60 seconds or less during casting, and at a temperature of 530 ° C. to 580 ° C. for 2 to 5 minutes And a high tensile strength of 3.5 kgf / mm ² was applied to the cold-rolled coil for 1 to 3 minutes at a temperature of 1180 ° C. to 1220 ° C. at a target annealing temperature of 10 ° C./sec. When the heat treatment is performed under stress, the intermetallic compound is completely decomposed to obtain a good microstructure, but the tensile stress applied to the strip in the annealing furnace is too high to cause breakage of the coil plate.

In the case of Inventive Example 1, the strip is heat-treated at a temperature of 1230 ° C to 1250 ° C for 15 to 60 seconds or less during casting, and then a hot-rolled coil is produced. The hot rolled coil is annealed at a temperature of 530 ° C to 580 ° C for 2 to 5 minutes And a tensile stress of 2.0 kgf / mm ² was applied to the cold-rolled coil for 1 to 3 minutes at a temperature of 1180 ° C. to 1220 ° C. at a target annealing temperature of 10 ° C./sec. It can be confirmed that a cold-rolled coil having excellent corrosion resistance can be produced if the intermetallic compound is completely decomposed as shown in Fig. 5 to obtain a good microstructure.

In the case of casting 2, the strip is heat-treated at a temperature of 1230 ° C to 1250 ° C for a time within 15 to 60 seconds, and a hot-rolled coil is manufactured. When the cold rolled material is annealed, the temperature is 530 ° C to 580 ° C within 2 to 5 minutes After applying the residual stress relieving heat treatment for a period of time, tensile stress of 3.0 kgf / mm ² is applied to the cold-rolled coil for 1 to 3 minutes at a temperature of 1180 ° C. to 1220 ° C. at a target annealing temperature of 10 ° C./second It can be confirmed that a cold-rolled coil having excellent corrosion resistance can be produced if the intermetallic compound is completely decomposed under the condition that the coil plate breakage does not occur to obtain a good microstructure.

Meanwhile, in the present invention, the hot-rolled coil is manufactured by strip casting at a temperature of 1230 ° C to 1250 ° C for 15 to 60 seconds in the strip annealing condition before the intermediate rolling process. However, the strip annealing condition can be maintained at 1250 ° C or more If the strip heat treatment conditions exceed 1250 占 폚, there is a problem that equipment load and economical efficiency are lowered.

It is also preferable that the heating rate is at least 10 ° C / second, but it is also possible to give a heating rate higher than that shown in FIG. However, if the temperature raising rate is increased too much, the economical efficiency is lowered. Therefore, when the economical efficiency is considered, the upper limit value is preferably 20 ° C / second.

Therefore, in the super-austenitic stainless steel manufacturing method according to the embodiment of the present invention, the super-austenitic stainless steel (S32050 steel) is heated by induction heating in the strip casting process before the intermediate rolling process (intermediate rolling mill) Is maintained at a temperature of 1230 to 1250 ° C for 15 seconds to 60 seconds to produce a hot-rolled coil. When the hot-rolled coil is cold-rolled after the cold-rolling, the annealing is preferably performed at a temperature of 530 to 580 ° C. Annealing treatment) for 2 minutes to 5 minutes and then raising the target annealing temperature to a target annealing temperature of at least 10 ° C / sec at a residual stress relieving heat treatment temperature, and then maintaining the annealing temperature for 1 minute to 3 minutes desirable. In this case, when a line tension applied to the cold-rolled coil at a target annealing temperature is applied in the range of 2.0 to 3.0 kgf / mm ² , the residual intermetallic compound decomposition due to the stress-induced transformation is promoted, It is possible to manufacture a coil.

Although the present invention has been described with reference to the accompanying drawings and the preferred embodiments described above, the present invention is not limited thereto but is limited by the following claims. Accordingly, those skilled in the art will appreciate that various modifications and changes may be made thereto without departing from the spirit of the following claims.

Claims (7)

Strip casting the molten steel with controlled components, heat treating the hot molten steel, and intermediate rolling (IRM) the heat-treated strip to produce a hot-rolled coil;
A step of subjecting the produced hot-rolled coil to cold-rolling and then subjecting the cold-rolled coil to primary annealing to remove residual stress;
Heating the cold-rolled coil subjected to the first annealing to a target temperature;
And a step of subjecting the cold-rolled coil subjected to the first annealing at the target temperature to a secondary annealing while applying an annealing tensile stress in a range of 2.0 to 3.0 kgf / mm < ^{2 &} gt ;.
The method according to claim 1,
In the step of producing the hot-rolled coil,
The molten steel has a carbon (C) content of more than 0 to 0.030 wt%, a silicon (Si) content of more than 0 to 0.5 wt%, a manganese (Mn) content of more than 0 to 0.6 wt%, a phosphorus (P) , Sulfur (S): more than 0.001 to less than 0.004 wt%, chromium (Cr): more than 22 to less than 24 wt%, nickel (Ni): more than 20 to less than 23 wt%, molybdenum (Mo) (N) of more than 0.21 to less than 0.32 wt%, balance of iron (Fe) and unavoidable impurities,
Wherein the value of the formula of the resistance index defined by the following formula 1 is 50 or more.
[Formula 1]
Formal resistance index (PREN) =% Cr + 3.3% Mo + 30% N
In Equation 1,% Cr,% Mo, and% N mean the content (wt%) of each component.
The method according to claim 1,
In the step of producing the hot-rolled coil,
Wherein the strip is maintained at a temperature range of 1230 to 1250 占 폚 for 15 to 60 seconds by induction heating before the intermediate rolling process.
The method according to claim 1,
In the step of subjecting the cold-rolled coil to primary annealing to remove residual stress,
Wherein the cold-rolled coil is held at a temperature range of 530 to 580 占 폚 for 2 to 5 minutes.
The method according to claim 1,
In the step of raising the temperature of the cold-rolled coil subjected to the primary annealing to the target temperature,
Wherein the temperature is elevated to a target temperature of 1180 to 1220 占 폚 while the heating rate is 10 占 폚 / sec or more.
The method of claim 5,
In the step of raising the temperature of the cold-rolled coil subjected to the primary annealing to the target temperature,
And the rate of temperature rise is 20 占 폚 / sec or less.
The method according to claim 1,
In the step of secondary annealing at the target temperature,
Wherein the cold annealed coil subjected to the primary annealing is held for 1 to 3 minutes at a target temperature range of 1180 to 1220 占 폚.

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